using a correlated subquery in update

I have a table that has a column that has values like XM_29832.11 and I
want to do an update to take off the .11 part. I can do this for a
single value using:

select substring('XM_29832.11' from '^([A-Z]*_[0-9*])');

However, how can I write an update to use the above as a subquery to
update the whole column at once?

Thanks,
Sean